1. Field of the Invention
This invention relates to a drive circuit arrangement for a gas discharge lamp, comprising a self resonating inverter for supplying a drive voltage across a load having an impedance with a reactive component, the inverter having a pair of field effect transistors which operate in anti-phase iri a first resonant circuit. It relates particularly, though not exclusively, to a drive circuit arrangement for inductively coupled backlights for displays, hazard beacons or vehicle signals.
2. Background of Related Art
Inductively coupled backlights having air cored coils must be driven at frequencies in the Megahertz range to avoid excessive coil losses. At these frequencies the gate impedance of suitable field effect transistors (FET's) is low, so that the gate drive currents are high in consequence. To minimize the drive power a resonant tank circuit may be provided which cancels the reactive component of the gate impedance at the driving frequency, and makes the gate input impedance look resistive.
A difficulty with known drive circuits occurs when the inverter is supplied by a d.c. voltage which is not constant--for example when supplied from a vehicle battery or the like. It has been observed that optimum circuit efficiency is not obtained under such conditions.
EP-A-0 696 157 discloses a low frequency ballast circuit for an arc discharge lamp. A difficulty with such known drive circuits relates to the loop phase shift of self-oscillating circuits having integral loads. A stable oscillator has a loop phase shift of either 0 or a multiple of 2.pi.. Since any combination of lossless components has a phase shift of either 0 or .pi., and since the load, being resistive, contributes a phase shift which is neither 0 nor .pi., an extra phase shift is required which may be, for example, .pi./4. A problem is how to provide this phase shift without incurring extra losses.
A third difficulty relates to discharge lamps as loads in a resonant circuit. Before the discharge strikes the lamp acts as a high impedance and the resonant circuit will have a high Q factor. The current and voltage from this resonant circuit can cause damage circuitry.